import java.util.Scanner;

public class Main1608 {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int x=a%10;//ge        5
        int y=(a/10)%10;//shi  3
        int z=a/100;//bai      1
        double b=(z+y*1.0)/(y+x) ;
        System.out.println(String.format("%.2f",b));
    }
}
